In the operation of centrifugal clutches, frictional linings are used which can sustain high mechanical and thermal loadings and suffer as little wear as possible in this process. These clutches are used in hydrodynamic torque converters as bridging clutches. See German Offenlegungsschrift No. 2,743,595. During extensive investigations of such centrifugal clutches, it has been found that frictional linings similar to those used, for example, in automotive vehicle transmissions, do not produce satisfactory results for differing reasons.
Semi-metal linings (asbestos/binder/metal chips), for example, have too much wear and, in addition, heavy rattling (stick-slip effect) occurs in the synchronization region.
The behavior of metal linings is substantially better regarding their ability to resist mechanical and thermal load. But, they are also subject to a slight stick-slip effect during the synchronization phase. The very low coefficient of sliding friction has also an unfavorable effect so that long slip periods can occur during certain operating conditions with an associated increase in heat release. One outstandingly favorable property of the metal lining, however, is that it has a lower coefficient of static friction with impact loading parallel to the friction surface compared with that found when the load increases steadily. This has the effect that in the event of a shock torque, which may be caused by a change of gear in an automatic transmission, the clutch opens briefly and the shock is thus reduced by the resulting slip. In the case of a steady or slowly increasing torque being applied, as during an acceleration phase for example, a higher torque can be transferred.
Paper linings (based on asbestos fiber with a binder and linings manufactured on paper machines) behave perfectly with respect to the stick-slip effect, as might be expected. In contrast to semi-metal and metal linings, they have the property that their coefficient of sliding friction is greater than their coefficient of static friction and this produces smooth and shock-free behaviour during the synchronization phase. Unfortunately, such frictional linings are not adequate to deal with the high energy and mechanical loads. When used in a centrifugal clutch, unfavorable irreversible changes soon occur to the properties of the lining.
Thus, there is a need to find a frictional lining for a wet clutch or brake, which combines the favorable properties of a paper lining with those of a metal lining. These favorable properties are rattle-free, smooth behavior in the synchronization phase, the ability to deal with high energy and mechanical loads, extremely small wear, high resistance to the liquids used in high performance transmissions (for example, ATF oils), higher coefficient of friction in the sliding phase before synchronization compared with that after synchronization has been completed and lower coefficient of friction with impact loads compared with that occurring under steady load after synchronization has been completed and lower coefficient of friction at high surface pressures. The present invention seeks to overcome the aforementioned disadvantages of prior art linings.
These and other objects, features and advantages of the present invention will become more apparent form the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, embodiments in accordance with the present invention.